1. Technical Field of the Invention
The present invention relates to liquefied natural gas (LNG) processes. More specifically, this invention relates to offshore LNG production on nautical vessels for stranded gas reserves.
2. Description of Prior Art
Natural gas in its native form must be concentrated before it can be transported economically. The use of natural gas has increased significantly in the recent past due to its environmentally-friendly, clean burning characteristics. Burning natural gas produces less carbon dioxide than any other fossil fuel, which is important since carbon dioxide emissions have been recognized as a significant factor in causing the greenhouse effect. LNG is likely to be used more and more in densely-populated urban areas with the increased concern over environmental issues.
Abundant natural gas reserves are located all over the world. Many of these gas reserves are located offshore in places that are inaccessible by land and are considered to be stranded gas reserves. Reserves of gas are being replenished faster than oil reserves, making the use of LNG more important to the future. In liquid form, LNG occupies 600 times less space than natural gas in its gaseous phase. Since many areas of the world cannot be reached by pipelines due to technical, economic, or political reasons, using nautical vessels to transport LNG is an ideal choice.
Various schemes have been developed through the years to allow production of gas in the stranded gas reserves. Most schemes consisted of laying out a traditional LNG processing unit on the top of a dedicated floating barge or nautical vessel that was specifically built for the floating LNG process. However, most previous attempts have been cost prohibitive due to the logistics involved in such a process and the expense of a custom made nautical vessel. In addition to the high costs that average USD $180 million for a typical LNG carrier, the extremely long lead times of around three years required to manufacture a custom nautical vessel also adds considerable time and costs to the production projects.
In U.S. Pat. No. 6,003,603, Breivik teaches the use of two ships for the processing and storage of offshore natural gas. The first ship includes the field installation for gas treatment. The treated gas is then transferred in compressed form to an LNG tanker for conversion to a liquefied form, which is stored on the LNG tanker. Breivik utilizes a single refrigerant for cooling purposes within the liquefaction process, which is either in a liquid phase or a mixed phase. Once the LNG tanker storage vessels are full, the LNG tanker is disconnected from a buoy to which it is attached and sets sail. Another LNG tanker takes its place to receive the treated inlet gas for liquefaction. The LNG tanker is required to be seaworthy in order to transport the LNG product from the stranded reserves to facilities for further use.
A need exists for a more economical and efficient method of producing gas in the stranded gas reserves. It would be desirable to use existing nautical vessels, which are readily available and are not as expensive as the custom nautical vessels of the prior art. It would be advantageous for the LNG liquefaction process unit to be relatively compact to enable the process to be installed upon a nautical vessel. It would be advantageous to provide a process apparatus for exploitation and liquefaction of natural gas offshore in the stranded gas reserves through the use of existing nautical vessels.